1. Field of the Invention
The present invention relates to polyester resins suitable for the production of plastic containers, such as beverage bottles. Specifically, the present invention is directed to the use of a preblended additive in polyester synthesis reactions to reduce or eliminate unwanted color and/or improve bottle clarity, and to compositions including such preblended additive. More specifically, a cobalt/phosphorus compound or complex is formed and added at the end of esterification or at the beginning of polycondensation during polyester production.
2. Background of the Invention
Polyester resins are extensively used in the packaging industry and especially in the production of plastic bottles for containing a wide variety of liquids, including carbonated and non-carbonated beverages. In the manufacture of bottles, such polymeric resins are extruded and formed into chips. The use of chips in injection molding to make a bottle preform which then is reheated and blown into a mold to produce the final shape of the bottle, is well-known and established in the industry.
One problem that is often encountered in the bottle manufacturing process is the occurrence of a yellow color in the final polyester product. The yellow color in polyester articles results from undesirable side reaction products in the esterification and polycondensation reactions, which impart such yellow color. The yellow color of the polyester resin has resulted in the widespread usage of cobalt compounds as blueing agents in the manufacturing process. Blueing agents counterbalance the yellow color to produce a neutral bottle color. Cobalt acetate is the most common compound and when added in the range of 30-70 ppm (parts per million, weight) is generally sufficient to cover the yellowness in the resin or bottle. The use of cobalt as a blueing agent is more fully described in U.S. Pat. No. 5,623,047.
Although cobalt compounds are very effective against the yellowness problem sometimes cobalt can impart a xe2x80x9cblue hazinessxe2x80x9d to the product, which also is undesirable in the bottle manufacturing process. Microscopic analysis revealed that the haziness corresponded with the presence of particles 500 nanometers in diameter. The particles were identified as cobalt compounds or complexes.
In addition to using cobalt compounds as bluing agents, cobalt can also be used as an esterification catalyst to produce polyester resin, as described in U.S. Pat. Nos. 5,623,047 and 5,782,963. While it is generally recognized that cobalt can reduce the reaction time (enhancing the reaction rate) as an esterification catalyst, use of cobalt can yield resins of low clarity as described in U.S. Pat. No. 5,116,938, due to the aforementioned blue haziness characteristic. Additionally, the presence of cobalt in the polyester process may hinder the catalytic activity of manganese and lithium, which are often used as catalysts in the esterification reaction.
More specifically, polyethylene terephthalate (PET) is typically produced for the manufacture of plastic containers by the so-called DMT (dimethyl terephthalate) process or the TA (terephthalic acid) process. In the conventional DMT process, DMT and ethylene glycol (EG) are used as raw materials. The DMT and EG are reacted in a reaction vessel in the presence of an esterification catalyst at high temperatures, typically around 250xc2x0 C. The esterification catalyst can be any of various elements including manganese, zinc, calcium and cobalt. The esterification reaction produces the monomer and oligomers. It usually is necessary to arrest the activity of the esterification catalyst in the DMT process to prevent degradation of the resultant polymer and minimize further yellowness. It is known to add phosphorous compounds at the end of the esterification to sequester the catalyst to arrest their catalytic activity.
In the conventional TA process, TA and EG are reacted in a vessel without the addition of catalysts and this direct esterification occurs at high temperatures of about 250xc2x0 C. to produce monomer and oligomers. The monomer and oligomers are then polymerized by polycondensation under high temperature (250 to 300xc2x0 C.) and vacuum to produce PET.
It is a primary objective of the present invention to provide polyester resins that are transparent (i.e., have no haziness), and are substantially neutral in color (i.e., have no significant yellowness or blueness) and are suitable for bottle production.
Other objects, features and advantages of the invention will be more fully apparent from the following disclosure and appended claims.
The present invention provides polyester resins that are transparent (i.e., have no haziness), and have no significant yellowness or blueness, by use of a preblended cobalt-phosphorus complex or compound in the polyester synthesis reaction.
More specifically, the present invention provides an additive composition and process for the production of PET by a dimethyl terephthalate (DMT) or terephthalic acid (TA) process, wherein the final PET resins have good clarity and color characteristics. The PET resins hereinafter more fully described are employed for the manufacture of plastic containers such as bottles. The invention utilizes an improved additive that stabilizes the cobalt and substantially eliminates the xe2x80x9cblue hazexe2x80x9d which frequently results from the addition of cobalt in the esterification or polycondensation reaction. Additionally, the present invention yields a resin having good color and little or no haziness.
In application to bottle production, the polyethylene terephthalate produced in accordance with the present invention, by use of the preblended cobalt formulation, yields a clear non-hazy bottle, which exhibits minimal or attenuated color (yellow or blue).
In one aspect, the invention relates to a method of making a PET resin that is further polymerizable by solid-state polymerization and moldable to form a PET article that is transparent and substantially free of hazy and yellow or blue coloration, comprising producing a PET resin by a DMT or TA process, including formation of the monomers, and subjecting the monomers to polycondensation reaction to form PET resin, wherein before the polycondensation reaction, an effective amount of a complex or compound comprising a cobalt compound and a phosphorus compound are added to the reaction vessel, to form a resin that is further polymerizable by solid-state polymerization and moldable to form a PET article that is transparent and substantially free of yellow or blue coloration.
In another aspect, the present invention relates to a method of making a PET resin, comprising the steps of:
reacting dimethyl terephthalate (DMT) or terephthalic acid (TA) with ethylene glycol (EG), in the presence of a catalytically effective amount (if needed) of an esterification catalyst, such as manganese acetate, zinc acetate, or cobalt acetate, to produce monomers;
mixing the monomer with a preblended additive solution comprising cobalt acetate and polyphosphoric acid (PPA) to form a preblended monomer composition; and
subjecting the preblended monomer composition to a polycondensation reaction in the presence of a polycondensation catalyst such as antimony trioxide, antimony acetate, titanium butylate, titanium glycolate or titanium oxalate.
Another aspect of the invention relates to a method for mitigating color and haze development in the manufacture of bottles molded from a PET resin, said method comprising the steps of:
reacting a dicarboxylic acid or its ester equivalent with a diol at a temperature from about 150xc2x0 C. to about 270xc2x0 C. to start the reaction and form monomer;
preparing an additive complex or composition comprising a cobalt compound and a phosphorus compound;
adding the additive complex or composition before the polycondensation reaction;
subjecting the monomer and additive composition to a polycondensation reaction in the presence of a polycondensation catalyst at a temperature in the range of from about 250xc2x0 C. to about 300xc2x0 C., to form the PET resin; and
molding the PET resin to form a bottle of mitigated color and haze development.
The invention relates in a further aspect to a cobalt-phosphorus complex or compound for mitigating the development of color and haze in an article molded from a polyester resin, such cobalt-phosphorus complex or compound comprising a cobalt compound and a phosphorus compound.
Another aspect of the invention relates to a preblended additive comprising cobalt acetate and polyphosphoric acid (PPA), useful in the above-described PET process.
Yet another aspect of the invention relates to a molded PET article formed of a PET resin produced by a DMT or TA process, wherein the PET resin has been treated by a cobalt-phosphorus complex or composition comprising the reaction product of a cobalt compound and a phosphorus compound.
Other aspects, features and embodiments of the invention will be more fully apparent from the following disclosure and appended claims.